Most liquid crystal displays suffer from perceptually significant color errors. The spectral selectivity of liquid crystal layers is one of the origins responsible for wrong color rendering and grayscale coloring in liquid crystal displays.
The operation principle of a liquid crystal display requires a polarizer. The function of the polarizer is to selectively transmit or reflect light with a preferred direction of polarization. The unpolarized light transmitted through (or reflected by) linear polarizer has a polarization direction collinear with the so-called transmission axis of the polarizer.
The polarizing capability of a linear polarizer is characterized by the dichroic ratio. In reality, a small part of light with a polarization vector perpendicular to the transmission axis may be transmitted through a polarizer. Therefore, the transversal absorption coefficient (k⊥) has a high but finite value. A small part of light with polarization vector parallel to the transmission axis may be absorbed by the polarizer, thus the longitudinal absorption coefficient (k∥) has a relatively small non-zero value. The dichroic ratio is defined as:
                              K          d                =                              k            ⊥                                k            ∥                                              (        1        )            
A high dichroic ratio means a high degree of polarization of the light transmitted through a polarizer.
Another important quality of a polarizer is the spectral dependency of the dichroic ratio. The transversal and longitudinal absorption coefficients are dependent on the wavelength of light. Therefore, the dichroic ratio is also wavelength-dependent. This dependence reveals itself in the coloration of the initially white light passed through the polarizer. The sample transmission spectrum of two perpendicularly crossed typical polarizers is shown in FIG. 1. The spectrum reveals a progressively increasing spectral leakage below about 550 nm, and a large, rapidly increasing leakage in the long wavelengths above approximately 680 nm. These leakages result in perceptible coloration of the polarizer. However, it should be noted that the sensitivity of the human visual system to wavelengths of 680 nm and above is extremely low and that most sources of illumination for LCDs have minimal intensity in this region. As a result the predominant source of coloration due to polarizer leakage is the short wavelength region.
The described coloration can take place in different types of polarizers. The value of the coloration depends on the particular type of the polarizer, but remains perceptible to the human eyes. The spectrum shown in FIG. 1 is characteristic of iodine-based polarizers. The iodine-based polarizers are widely used in liquid crystal displays due to their relatively high dichroic ratio. Other types of polarizers, including dichroic dye-based ones, are also subject to coloration.
The described polarizer coloration is one of the reasons of coloration of liquid crystal cells. The magnitude and significance of color errors and color variations will vary with the particular optical configuration of liquid crystal cells and display applications. In some cases, even relatively large color errors and variations may be well tolerated by consumers of low-cost monochrome liquid crystal displays. However, for color liquid crystal displays in general and high-performance, full-color active-matrix liquid crystal display panels (AM LCD) in particular, users have come to expect a level of color accuracy and stability commensurate with the high-quality color cathode ray tube displays in today's televisions and computer workstation monitors. The designated high level of liquid crystal cell color performance requires elimination of virtually all color errors and variations, including the coloration introduced by the polarizer.
In particular, the origin of color errors and color variations of liquid crystal cells can be traced to two principal causes: shifts in the peak of spectral transmission or reflection resulting from changes in the effective birefringence of the liquid crystal layer and phase retardation between polarization components, and departures from ideal polarization performance in real polarization control films as described above as coloration of the polarizer. The first source of color errors typically dominates at high gray levels and can often be effectively managed by reducing the birefringence and/or thickness of the liquid crystal layer. On the contrary, the polarizer-related coloration dominates at low gray levels and persists down to the black level of the display.
Accordingly, a simple, polarization-sensitive color correction applicable for both polarizers and liquid crystal cells is desirable. It is also desired to provide a color correcting means with high transparency in the visible wavelength region to retain high transparency of the polarizer or liquid crystal cell.